A novel method for displacement measuring is proposed based on the bifurcate subcavity modulation effect in a dual
frequency laser. Owing to the intracavity wave plate, the laser operates on two orthogonally polarized modes. If a
portion of laser, channeled into a subcavity by the wave plated, is reinjected back to the resonator, the mode intensities
are modulated with a period of a half of wavelength, respectively. And the two modulations behave a 90 degree phase
difference, which could subdivide a period into four regions: horizontal polarization, two polarizations, vertical
polarization, and no light. In this paper, this subcavity modulation effect is demonstrated experimentally and
theoretically. Furthermore, a displacement measurement of the subcavity mirror is developed with resolution up to λ/8,
viz. about 79nm at laser wavelength of 632.8nm. The direction discrimination of measuring could also be realized
utilizing the sequence of four different subdivision zones. The measuring range, mainly limited by both the splitting
frequency difference and the bifurcate subcavity length, could reach tens of millimeters. Measurement results are
presented and the corresponding error analyses are discussed. This system has great potential in applying to
metrological fields for its simplicity, compactness and high quality performance.
The principle for utilizing a full-external-cavity He-Ne laser as a displacement sensor is presented. Inserting a quartz plate into the cavity, we split one laser frequency into two orthogonally polarized beams (o light and e light). When one cavity mirror is moved along the laser axis, we obtain power-tuning curves for o light and e light, in which equal-intensity points appear periodically, one period corresponding to /2 displacement. Moreover, four different polarization states in the laser output appear periodically. Attaching the moving mirror to the measured object, we realize displacement measurement with resolution of /2 through counting the number of equal-intensity points. We measure displacements less than /2 by means of a PZT mounted to the other cavity mirror. An increasing voltage is applied to the PZT to displace the mirror. Once the two beams reach the neighboring equal-intensity point, we note down the voltage variation of the PZT to get the true displacement. We discriminate displacement directions by sensing the order of appearance of the four polarization states. This system can be expected to afford a measurement range of 35 mm and a resolution of 10 nm, and has the capability of self-calibration. The potential error factors are also discussed.
A measurement system utilizing laser frequency splitting is presented for the precise measurement of retardations of arbitrary wave plates. The wave plate to be measured is inserted into a laser resonance cavity, and its retardation can be determined by measuring the difference of split frequencies and longitudinal mode spacing of the laser. In the system, mode spacing measurement is performed under multimode oscillation according to the relation between polarization directions of modes. To solve the problem of strong mode competition in measuring half-wave plates, a transverse magnetic field is applied to weaken the competition so as to make the system applicable even when the frequency difference is less than the threshold for strong mode competition. The error of the system is less than 5 arcmin, and standard deviations of about 1 arcmin are obtained from repeated experiments samples of quarter- and half-wave plates. No element rotation is required during measurement, and no mechanical quantity, such as angle, needs to be measured.
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